The present invention relates to an apparatus for and a method of diagnosing an exhaust gas recirculation (EGR) system for an internal combustion engine of a motor vehicle.
The EGR systems usually have an exhaust gas recirculation (EGR) passage with an exhaust gas recirculation (EGR) control valve. A portion of exhaust gas discharged from an internal combustion engine through an exhaust system is admitted into the EGR passage and recirculated under the control of the EGR control valve to an intake system of the engine, suppressing the maximum combustion temperature within the engine, resulting in a reduction in production of NOx within the engine. This portion of exhaust gas may be referred to as an EGR gas. However, if an amount of EGR gas is too abundant, the combustion within the engine is deteriorated, degrading stability of the engine operation, resulting in a reduction in output of the engine. The amount or flow rate of EGR gas should be determined in coordination with an air-fuel ratio and an ignition timing. Accordingly, a control unit is provided to control the amount of EGR gas with an air-fuel ratio control and an ignition timing control in accordance with running conditions of the engine. In controlling the amount of EGR gas, it is important to know accurately the operating state of the EGR system for improving control precision of the EGR system and/or diagnosis thereof.
Japanese Preliminary Patent Publication No. 51-94025 discloses an apparatus includes a temperature sensor such as thermistor in an EGR passage and it judges whether the EGR system is in trouble or not in response to a change in temperature within the EGR passage. Specifically, a change in the temperature is used as the criteria in making a judgment whether the EGR gas is actually recirculated in engine running conditions where the EGR is demanded. If the presence of the EGR is not detected, the apparatus determines that the EGR system is in trouble and gives some alarm to a driver. Engine running conditions where a diagnosis of the EGR system is made, namely a diagnosis area, are defined in terms of a basic pulse width Tp of fuel injection pulse, which is determined in accordance with an intake air quantity and a number N of revolutions of the engine (i.e., engine speed).
Referring to FIG. 3, an area enclosed by a one-dot-chain line represents engine running conditions where the EGR is to be effected, while a shadowed area represents engine running conditions where diagnosis is made. This shadowed area is defined in terms of the basic pulse width Tp and the number N of revolutions of the engine. Assuming now that the engine running conditions shift from a point A to a point B, a temperature change within the EGR passage is large. However, on shifting from a point C to the point B, the temperature change is small because EGR has been already applied during operation at the point C. This means that a value of the temperature change used for the diagnosis has to be set small, so that a high precision is demanded on the apparatus for diagnosing the EGR system. In other words, if the diagnosing apparatus is not of high precision required, accurate diagnosis cannot be made.
Another known diagnosing apparatus is disclosed in Japanese Preliminary Patent Publication No. 60-21374, which includes a temperature sensor in an EGR passage and determines that the EGR system is in trouble when the detected temperature within the EGR passage is lower than a predetermined value.
With this known apparatus, whether or not the EGR system is in trouble is judged on the result of comparing the detected temperature with EGR passage and a predetermined value. Accordingly, under a condition where the atmospheric temperature is low as shown by a broken line drawn curve in FIG. 4, the diagnosing apparatus exhibits a poor response owing to a temperature drop of the EGR gas caused by the low atmospheric temperature. As a result, the rise in temperature of the EGR gas up to the predetermined value is considerably delayed. Thus, taking this situation into account, a long waiting time is set in this known diagnosing apparatus. Since the occurrence of fault or malfunction of the EGR system can not be detected during this waiting time, it is demanded that the waiting time be short.
An object of the present invention is to provide an apparatus for and a method of diagnosing an EGR system with a better accuracy.